Heretofore, various types of fuel injectors and fuel injection systems have been known in the prior art which are applicable to internal combustion engines. Of the many types of fuel injection systems, the present invention is directed to unit fuel injectors, wherein a unit fuel injector associated with each cylinder of an internal combustion engine and each unit injector includes its own drive train to inject fuel into each cylinder on a cycled basis. Normally, the drive train of each unit injectors is driven from a rotary camshaft operatively driven by the engine crankshaft for synchronously controlling each unit injector independently and in accordance with the engine firing order.
Of the known unit injectors, there are two basic types of unit injectors which are characterized according to how the fuel is metered and injected. The first type, which is that type to which the present invention is directed, is known as "an open nozzle" fuel injector, in that the fuel is metered through a metering chamber within the unit injector where the metering chamber is open to the engine cylinder by way of an injection orifice during fuel metering. In contrast to the open nozzle type injector, there are also unit injectors classified as "closed nozzle" injectors wherein fuel is metered to a metering chamber within the unit injector while the metering chamber is closed to the cylinder of an internal combustion engine by a needle tip valve mechanism that is opened only during injection by increasing the fuel pressure acting thereon.
In either case, the unit injector typically includes a plunger element that strikes the metered quantity of fuel to increase the pressure of the metered fuel and force the metered fuel into the cylinder of the internal combustion engine. In the case of a closed nozzle injector, a tip valve mechanism is provided for closing the injection orifice during metering where the tip valve is biased toward its closed position to ensure that injection will take place only after fuel pressure is increased sufficiently to open the tip valve mechanism. The present invention is directed to the open nozzle type fuel injector and more particularly to a unit injector fuel injection system that relies on pressure and time principles for determining the quantity of fuel metered for each subsequent injection of each injector cycle. Moreover, the pressure-time principles allow the metered quantity to be varied for each cycled operation of the injector as determined by the pressure of the fuel supply to the metering chamber and the time duration that such metering takes place. Examples of such injectors of the open nozzle type are described in detail in U.S. Pat. No. 4,280,659 issued to Gaal and U.S. Pat. No. 4,601,086 issued to Gerlach, each of which are assigned to the assignee of the subject invention. Each of the injectors disclosed therein include a plunger assembly with a lower portion having a major diameter section that is slidable within an axial bore of the injector body and a smaller minor diameter section that extends within a cup of the injector body. The cup provides an extension of the axial bore which is smaller in diameter than the diameter of the axial bore that passes through the remainder of the injector body. During the metering stage, fuel is metered through a supply port into the axial bore at a point above the cup and the fuel flows around the minor diameter section of the plunger assembly at a tip thereof, thus metering a specific quantity of fuel into the metering chamber of the cup. A radial gap is provided between the minor diameter section of the plunger assembly and the inner wall of the bore within the cup. This gap facilitates the flow of fuel into the injector tip to be injected. Once the metering stage is completed, the plunger travels inwardly (defined as toward the engine cylinder of an internal combustion engine) so as to cause injection of the fuel from the metering chamber through the injector orifice.
The stage just after fuel injection is completed is known as the crush stage, wherein the plunger tip is held tightly against a seat of the cup by an associated drive train of the unit fuel injector. During this crush stage, fuel is trapped within the radial gap between the minor diameter section of the plunger and the inner wall of the bore within a cup. This quantity of fuel is known as the trapped volume. It has been determined that this trapped volume results in the presence of higher levels of unwanted emissions and particularly unburned hydrocarbons in the exhaust gas of an internal combustion engine. The increase in unburned hydrocarbons found in the emissions of the internal combustion engine is due to the tendency of the fuel within the trapped volume to migrate into the engine cylinder after combustion has occurred in the cylinder with such fuel subsequently being exhausted therefrom.
As can be understood from the above, such a problem is unique to open nozzle type fuel injectors, in that closed nozzle fuel injectors rely on a valve mechanism to seal the fuel from the engine cylinder at all times except during injection. Moreover, open nozzle injectors must allow the metering of fuel within the nozzle tip which includes injection orifices that are opened to the engine cylinder.
In an effort to overcome the above mentioned deficiencies, it has been proposed to reduce the trapped volume surrounding the minor diameter section of the plunger within the cup after injection. From the above noted prior art, the only suggestion is to simply reduce the radial gap between the minor diameter section of the plunger in the cup to thus reduce the trapped volume after injection is completed. However, such a modification becomes unacceptable and results in the insurmountable problem of no longer having a sufficient gap for the fuel to be metered into the nozzle area of the cup since the fuel flow around the minor diameter section of the plunger becomes significantly reduced as the gap is reduced. Specifically, it has been found that the quantity of metered fuel to be injected is reduced to a degree that insufficient fuel is injected into the cylinder. Therefore, such a solution is impracticable and unacceptable.
In addition to the foregoing, the components of the injector, specifically the plunger minor diameter section and the inner surface of the bore within the cup become carboned during usage of the unit fuel injector in an internal combustion engine from hot gases within the engine cylinder that are forced back into the injector. Furthermore, as carbon builds up on the minor diameter section of the plunger and the inner wall of the cup, the gap between the minor diameter section of the plunger and the inner wall of the cup continuously decreases over time. Accordingly, the gap must be sized so that even after carboning, a sufficient flow of fuel can be provided through the gap for adequate fuel metering.
It is clear from the above, that the above teachings to reduce trapped volume and to permit fuel metering without effect from injector carboning are in direct conflict with one another. That is, reducing the trapped volume would direct one to decrease the gap between the minor diameter of the plunger and the cup inner wall while reducing the sensitivity to fuel metering after carboning requires the gap size to be increased. The end result of the known open nozzle type unit injector technology is that the above noted goals must be balanced with one another to provide a compromised open nozzle unit type fuel injector that has a gap that partially achieves both goals.
Thus, there is a need for an open nozzle unit fuel injector that can reduce trapped volume between the minor diameter of the plunger and the inner wall of the injector cup while still permitting sufficient fuel flow therebetween to accurately and effectively control the fuel quantity and reduce unburned hydrocarbons in the emissions. Moreover, there is a need to provide such an open nozzle unit fuel injector that will function accurately over the entire useful life of such an injector without adversely effecting fuel metering even after the plunger and cup surfaces become fully carboned.
An effort to achieve such goals is set forth in U.S. Pat. No. 5,042,721 issued to Muntean et al. and assigned to the assignee of the subject invention. Therein, an open nozzle unit fuel injector is disclosed for injecting a metered quantity of fuel into the cylinder of an internal combustion engine. The plunger of the open nozzle unit injector includes a major diameter section which is slidably moveable in an axial bore to open and close a fuel supply orifice and a minor diameter section that extends into the bore of a cup portion of the injector body. The cup portion has an internal surface including plural diameter portions connected by an annular step.
The fuel supply orifice is specifically located within the axial bore and the plunger minor diameter section is designed that such when the plunger is moved from its retracted position to its advance position, a portion of the minor diameter section becomes readily engaged or almost engaged with one of the plural cup surface sections before the major diameter section closes the fuel supply orifice. In doing so, a reduction in the buildup of carbon on the injector surfaces is achieved by reducing the time period during which gas can flow from the engine cylinder into the metering chamber of the unit injector through the injector orifices while maintaining a sufficient flow path for fuel into the metering chamber. However, even with the foregoing improvements, some trapped volume may still be present during the injection cycle.
Similarly, U.S. Pat. No. 5,037,03 1 issued to Campbell et al. and assigned to the assignee of the subject invention discloses an open nozzle fuel injector which includes modifications to either the minor section of the plunger or the cup of the injector housing in order to reduce the volume of fuel trapped during the forward stroke of the plunger assembly. However, as with U.S. Pat. No. 5,042,721, a trapped volume may still be present within the cup of the injector even after carboning of the surface of the plunger assembly and cup.
According, there is a need for an open nozzle unit injector that can reduce the trapped volume between the minor diameter of the plunger and the inner wall of the injector while still permitting fuel flow therebetween to accurately and effectively control the fuel quantity while reducing unburned hydrocarbons in the emissions of the internal combustion engine. Moreover, there is a need to provide such an open nozzle unit fuel injector which will function accurately over the entire useful life of the injector and one which the reduction and trapped volume is uniform across the several injectors of a respective internal combustion engine.